This application is based on Japanese Patent Application Nos. 2001-257618 filed on Aug. 28, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a torque converter provided with a lock-up clutch and having an engaging chamber and a releasing chamber and which is operable with a difference between fluid pressures in the engaging and releasing chambers.
2. Discussion of Related Art
A lock-up clutch is operable in a partially engaged or slipping state. An amount of friction heat generated by the lock-up clutch operated in the slipping state is increased with an increase in the operating range of the lock-up clutch in its slipping state (the range in which a transmission torque or slipping speed of the lock-up clutch placed in the slipping state is controllable) The lock-up clutch suffers from a problem of shortening of an expected service life of a friction member due to thermal deterioration during its operation in the slipping state. To avoid this problem, there have been proposed various arrangements effective to reduce a temperature rise of the friction surface of the lock-up clutch.
For example, JP-A-2-80857 discloses a lock-up clutch wherein a clutch piston is provided with a friction member attached thereto, and has a cooling passage formed in a radially inner portion thereof relatively close to its axis of rotation, so that heat generated as a result of frictional contact of the clutch piston with a front cover through the friction member is dissipated through a working fluid which flows through the cooling passage. JP-A-2001-132819 discloses a lock-up clutch wherein a front cover is provided with a friction member attached thereto, and the surface of a clutch piston on the side of the friction member has a cooling passage formed in a radially inner or central portion thereof located radially inwardly of a radially outer portion thereof for frictional contact with the front cover through the friction member, so that a working fluid in an engaging chamber of the lock-up clutch flows through the cooling passage in a radially inward direction, so as to effectively cool the heat-generating surface of the lock-up clutch. Thus, the known lock-up clutch is cooled at its heat-generating portion by the working fluid, so that the operating range of the lock-up clutch placed in its slipping state can be made relatively large.
The lock-up clutch disclosed in JP-A-2-80857 in which the friction member is attached to the clutch piston, the front cover which generates heat due to its slipping contact or engagement with the friction member must be cooled by the ambient air whose coefficient of thermal conductivity (heat conductivity) is lower than that of the working fluid. Accordingly, the front cover cannot be efficiently cooled.
On the other hand, the lock-up clutch disclosed in JP-A-2001-132819 in which the friction member is attached to the front cover, the clutch piston which generates heat due to its slipping contact or engagement with the friction member can be cooled by the working fluid with a comparatively high degree of efficiency. However, this lock-up clutch wherein the clutch piston has the cooling passage may suffer from a problem of fluid leakage from the engaging chamber in the presence of the cooling passage. Namely, the working fluid may flow from the engaging chamber into the releasing chamber through the cooling passage, resulting in a decrease in the difference between the fluid pressures in the engaging and releasing chambers. Further, the cooling passage is required to have a relatively large diameter to assure a sufficiently high rate of flow of the working fluid therethrough as the cooling fluid. However, an increase in the diameter of the cooling passage undesirably reduces the pressure difference between the engaging and releasing chambers, leading to a decrease in the transmission torque of the lock-up clutch. In this respect, the rate of flow of the working fluid through the cooling passage is limited by the required transmission torque of the lock-up clutch. Thus, this lock-up clutch suffers from a problem that the rate of flow of the working fluid through the cooling passage cannot be made high enough to dissipate the generated heat, where the amount of the generated heat is large due to a large amount of slipping of the lock-up clutch, even where an input torque of the lock-up clutch is relatively small.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide a torque converter incorporating a lock-up clutch whose heat-generating portion can be efficiently cooled.
The object indicated above may be achieved according to the principle of the present invention, which provides a torque converter provided with a lock-up clutch and including a pump impeller, a turbine runner, a turbine hub, a stator, a clutch piston rotatable with the turbine runner and cooperating with the pump impeller to define therebetween an engaging chamber, a front cover cooperating with the clutch piston to define therebetween a releasing chamber, and a friction member attached to one of the clutch piston and the front cover, and wherein the lock-up clutch is placed in a slipping state for slipping engagement of the clutch piston and the front cover with each other through the friction member, by a controlled difference between pressures of a working fluid in the engaging and releasing chambers, characterized in that: the friction member is attached to the front cover; the pump impeller and the stator cooperate with each other to define therebetween one of a first fluid passage and a second fluid passage both of which communicate with the engaging chamber, while the stator and the turbine hub cooperate with each other to define therebetween the other of the first and second fluid passages; and the lock-up clutch is brought into the slipping state with a supply flow of the working fluid into the engaging chamber through the first fluid passage and a discharge flow of the working fluid from the engaging chamber through the second fluid passage.
In the torque converter of the present invention constructed as described above, the friction member is attached to the front cover, so that the clutch piston which generates heat due to frictional slipping contact or engagement with the friction member during an operation of the lock-up clutch in its slipping state can be cooled by the working fluid flowing through the engaging chamber. Further, the working fluid is circulated from the first fluid passage to the second fluid passage through the engaging chamber during the operation of the lock-up clutch in the slipping state, so that the clutch piston can be efficiently and effectively cooled.
The supply flow of the working fluid from the first fluid passage into the engaging chamber and a discharge flow of the fluid from the engaging chamber through the second fluid passage are consistent with the fluid flow for transmission of a rotary motion from the pump impeller to the turbine runner during an operation of the torque converter under load, that is, follows the fluid flow from the pump impeller toward the turbine runner and stator, so that the clutch piton can be effectively cooled by the flowing fluid.
Further, the working fluid does not leak from the engaging chamber, the lock-up clutch operated in the fully engaged or lock-up state does not suffer from a decrease in the torque transmission capacity due to the fluid leakage during its operation in the fully engaged or lock-up state.
According to one preferred form of the present invention, the lock-up clutch is brought into a fully engaged state for full engagement of the clutch piston with the front cover through the friction member, with the supply flow of the working fluid into the engaging chamber through the first fluid passage, while the discharge flow of the working fluid from the engaging chamber through the second fluid passage is prevented.
In the torque converter according to the above-indicated preferred form of the invention, the working fluid is not discharged from the second fluid passage during an operation of the lock-up clutch in the fully engaged state wherein no heat is generated. Accordingly, reduction of the fluid pressure in the engaging chamber in the fully engaged state is prevented, permitting the lock-up clutch to maintain the nominal torque transmission capacity.
In one advantageous arrangement of the above-indicated preferred form of the invention, the torque converter further includes a lock-up clutch control valve operable to control the difference between the pressures of the working fluid in the engaging and releasing chambers, and wherein the lock-up clutch control valve has a first position for permitting the discharge flow of the working fluid from the engaging chamber through the second fluid passage when the lock-up clutch is placed in the slipping state, and a second position for preventing the discharge flow when the lock-up clutch is placed in the fully engaged state.
In the above-indicated advantageous arrangement of the invention, the lock-up clutch control valve prevents the discharge flow of the fluid from the engaging chamber through the second fluid passage during an operation of the lock-up clutch in the fully engaged or lock-up state, and therefore eliminates a need of providing a valve exclusively used to prevent the fluid from being discharged from the engaging chamber through the second fluid passage when the lock-up clutch is placed in its fully engaged state.
According to another preferred form of the present invention, the torque converter further includes: a flow control device operable to control a rate of flow of the working fluid into the engaging chamber through the first fluid passage; slipping-speed calculating means for calculating a slipping speed of the lock-up clutch which is a difference between rotating speeds of the clutch piston and the front cover when the lock-up clutch is placed in the slipping state; engine-torque estimating means for estimating a torque of an engine connected to the front cover; and flow-rate adjusting means for controlling the flow control device to adjust the rate of flow of the working fluid into the engaging chamber through the first fluid passage, on the basis of the slipping speed calculated by the slipping-speed calculating means and the torque of the engine estimated by the engine-torque estimating means, and according to a predetermined relationship between the rate of flow and the slipping speed and the torque of the engine, the predetermined relationship being determined such that the rate of flow increases with at least one of the slipping speed and the toque of the engine. In this form of the invention, the flow-rate adjusting means controls the flow control device such that the rate of flow of the fluid through the first fluid passage into the engaging chamber changes with the amount of heat generated by the lock-up clutch operated in the slipping state, so that the clutch piston can be efficiently and effectively cooled.